Various dielectric films have been formed in the past during the fabrication of semiconductor devices. For example, films such as silicon dioxide and silicon nitride have been used for dielectric films in the formation of capacitors, such as for memory devices including, for example, dynamic random access memories (DRAMs). With the shrinkage of minimum feature sizes of semiconductor devices, e.g., increase in memory cell density in DRAMs, there is a continuing challenge to maintain sufficiently high storage capacitance despite decreasing cell area. One way of increasing cell capacitance is through the use of different cell structures such as trench and/or stacked capacitors. However, as feature size continues to become smaller and smaller, development of improved materials for cell dielectrics, as well as the cell structure, have become important.
Conventional dielectrics such as silicon dioxide and silicon nitride may no longer be suitable for use in many devices because of their relatively small dielectric constants. Insulating inorganic metal oxide materials, e.g., ferroelectric materials and perovskite oxides, have gained interest for use as dielectrics in memory devices. Generally, these materials have high dielectric constants which make them attractive as dielectric materials in capacitors, for example, for high density DRAMs and other memory devices. As used in this document, a high dielectric constant refers to a dielectric constant of about 15 or greater. For example, such high dielectric constant materials include tantalum pentoxide (Ta2O5), barium-strontium-titanate (BST), strontium titanate (SrTiO3), barium titanate (BaTiO3), lead zirconium titanate (PZT), and strontium-bismuth-tantalate (SBT). Using such materials enables the creation of much smaller and simpler capacitor structures for a given storage charge requirement, enabling an increased packing density for memory devices.
The dielectric properties of such films are dependent on various film characteristics, such as the concentration of the components thereof, e.g., the concentration of titanium in a BST film. Further, certain high dielectric constant materials have better current leakage characteristics in capacitors than other high dielectric constant materials. In some materials, aspects of the high dielectric constant material might be modified or tailored to achieve a particularly high dielectric constant, which may unfortunately and undesirably also tend to hurt the leakage characteristics, e.g., increased leakage current. For example, with respect to metal oxides having multiple different metals bonded with oxygen, such as BST, PZT, and SBT, it is found that increasing titanium concentration of the components thereof results in different dielectric characteristics. For example, with respect to BST films, it is found that increasing titanium concentration as compared to barium and/or strontium results in improved leakage characteristics, but decreases the dielectric constant. Accordingly, capacitance can be increased by increasing the concentration of barium and/or strontium, but unfortunately at the expense of an increasing leakage current. Further, absence of titanium in the oxide lattice creates a metal vacancy in such multi-metal titanates which can increase the dielectric constant but unfortunately also increases the current leakage.
It is desirable to form such high dielectric constant films by chemical vapor deposition (CVD) at low deposition temperatures, i.e., less than 680° C. However, although step coverage is better at such low deposition temperatures, deposition rates for the high dielectric constant films is generally lower. Although an increase in deposition rate may occur at higher temperatures, such an increase in temperature over 680° C. may damage barrier materials used in conjunction with the high dielectric constant films.
Generally, at low deposition temperatures, incorporation efficiency of components in the film are affected. For example, relative to high deposition temperature processes for forming BST films, incorporation efficiency of titanium in the formation of such high dielectric constant films decreases in conventional low deposition temperature processes. In fact, the stoichiometry of the high dielectric constant films appear to be self-adjusting in low deposition temperature processes. In other words, changing precursor flow ratios does not affect film composition at lower deposition temperatures, unlike the significant effect such changing of precursor flow ratios has in high temperature CVD processes. For example, with respect to BST films, a change in precursor flow ratio (e.g., Ba/Sr to Ti ratio) does not substantially affect film composition of a deposited BST film at temperatures less than 680° C.
In many circumstances, it may be desirable to have varied concentrations within a high dielectric constant film (in other words, for example, changing the stoichiometry of different layers or portions of a BST film as it is deposited) deposited using low deposition temperature CVD processes. Since film composition is not affected by the conventional method of changing precursor flows, new methods of controlling the stoichiometry of high dielectric constant films are needed. Further, even if a film's stoichiometry is controlled to a certain degree by changing precursor flow, the control of stoichiometry by changing precursor flow is disadvantageous in that, for example, extensive time for conditioning is required to change such flows.